1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer using an electrophotographic mode or electrostatic recording mode. More particularly, the present invention relates to an image forming apparatus for forming a toner image for control in an inter-transfer-material area serving as an area on an intermediate transfer member between toner images to be transferred to a transfer member.
2. Description of the Related Art
In recent years, a technique corresponding to high image quality close to photographic quality or corresponding to high speed close to a printer is requested for an image forming apparatus using an electrophotographic image forming process. To achieve high speed and high image quality, maintenance of color stability and concentration uniformity becomes a problem. Therefore, a technique is widely used which forms a control image constituted of toner (toner image for control), detects the reflection concentration of the image, returns the reflection concentration to an image forming process condition and maintains a stable image.
The above control image is formed at the timing (sheet) corresponding to the portion between a transfer member for currently continuously forming an image for a plurality of transfer members and a transfer member. Therefore, it is necessary to remove (clean) the control image by cleaning means so that the image does not normally attach to an image forming object according to image formation.
To clean a control image from a photoconductor on which an image is directly formed or the surface of an image bearing member such as an intermediate transfer member, the following method is used. That is, the polarity of transfer bias is reversed to normal polarity when an image is formed by a transfer portion to the intermediate transfer member of an image or transfer portion to a transfer member. Thereby, it is possible to clean the control image by a cleaning member provided to clean each image bearing member without transferring a control image from an image bearing member such as a sensitive member or intermediate transfer member.
However, not to transfer a control image formed on a sheet in the flow to recent high-speed technique, it is very difficult to apply a bias having a polarity reverse to the normal polarity when forming an image in view of time and sheet distance.
When it is difficult to apply a reverse bias, a control image is transferred onto an intermediate transfer member from a photoconductor and moreover transferred from the intermediate transfer member to a secondary transfer member in intermediate-transfer-mode image formation. Therefore, when there is not a cleaning member for the secondary transfer member, a defective image due to back contamination or imperfect conveyance of a transfer member has been caused due to the contamination of the secondary transfer member.
Therefore, a member for cleaning a control image to be transferred to the secondary transfer member is necessary. As a cleaning member for the secondary transfer member, a blade mode having a high cleaning capacity is widely used in general. It is general that the secondary transfer member uses a member in which the traveling performance of a blade is stabilized by applying fluorine coating to the surface layer of the blade so that the cleaning capacity of the secondary transfer member is improved by the blade mode.
However, the secondary transfer member also frequently uses a member whose surface layer is roughened from the viewpoint of the conveying performance of a transfer member. In this case, it is possible to clean low-concentration toner attaching to a non-image portion in a developing step, for example, development-fogged toner even by the blade mode. However, to completely clean a high-concentration image such as a control image, it is necessary to increase a linear load at the nip portion between a secondary transfer member and a blade by increasing the contact pressure or contact angle of the blade. However, because the secondary transfer member and the cleaning blade are elastic bodies, frictional force is large. Therefore, when increasing the linear load at the nip portion between the secondary transfer member and the blade, there is a problem that toner is attached and burrs formed on the cleaning blade easily occur.
Japanese Patent Application Laid-Open No. 2002-229344 discloses an electrostatic cleaning mode for electrostatically removing the toner on an intermediate transfer member as a method for cleaning the toner left on an intermediate transfer member after a secondary transfer step. That is, the intermediate transfer member is rotated by bringing a conductive fur brush into contact with the intermediate transfer member. Then, a voltage applying member such as a metallic roller to which a voltage is applied is brought into contact with the conductive fur brush. Thereby, cleaning is performed by electrostatically adsorbing the toner on the intermediate transfer member (electrostatic fur-brush cleaning).
Japanese Patent Application Laid-Open No. 2000-187405 discloses a method for rumpling a toner image attached to a secondary transfer member by bringing a fur brush into contact with the secondary transfer member and rotating it.
Therefore, to clean a secondary transfer member whose surface layer is roughened, it is considered to use electrostatic fur-brush cleaning in which the number of restrictions on the surface shape of a member to be cleaned is small compared to the case of the blade mode.
Moreover, in the case of the electrostatic fur-brush cleaning, a bias having a polarity reverse to the polarity of toner is applied to a conductive fur brush through a voltage applying member such as a metallic roller. Then, by transferring the toner to the fur brush, cleaning is performed. In the case of this electrostatic fur-brush cleaning, even for a secondary transfer member whose surface layer is roughened, the front end of the fur brush enters up to a portion in which the surface of the secondary transfer member is roughened. Therefore, there is an advantage that it is possible to preferably perform cleaning.
However, as a result of studying cleaning of a secondary transfer member according to electrostatic fur-brush cleaning, it is found that there is the following problem.
To stabilize an image, the frequency for forming a control image represented by an image for concentration control may be increased by forming the control image every sheet. Moreover, to improve a read accuracy by a sensor, a control image having an area as large as possible may be formed. In this case, it is difficult to frequently clean high-concentration toner through electrostatic fur-brush cleaning and toner may clog in the fur of the fur brush. Thereby, the function of the fur brush cannot be sufficiently demonstrated and imperfect cleaning causing back contamination of a transfer member or image defect may occur at the time of both-side printing.
That is, in this case, when removing a toner image to be repeatedly transferred to a secondary transfer member at a predetermined cycle by a fur brush rotating by contacting with the secondary transfer member, a portion removing the toner image from the fur brush removes the toner image from the secondary transfer member again at the next rotation. Therefore, it is considered that removal of the toner transferred to the secondary transfer member is not preferably performed.
In this case, to improve the cleaning property by electrostatic fur-brush cleaning, a fur brush may be rotated at a high speed. However, it is difficult to rotate an electrostatic fur brush at a high speed because there are many restrictions on space and cost since a toner suction apparatus is separately required due to a problem of toner fly. Therefore, it is preferable that the peripheral speed ratio between a secondary transfer apparatus and a fur brush becomes 1 or less.
By increasing a fur brush in diameter, it is considered to increase the contact area with a member to be cleaned and improve the cleaning property. However, a large diameter is difficult because of mechanical space.